1. Field of the Invention
The invention relates to a method and apparatus for manufacturing metallic parts, more particularly to a method and apparatus for manufacturing metallic parts by a process involving injection of a melted metal into a mold in a thixotropic state.
2. Description of the Related Art
One conventional method used to produce molded metallic parts from melted metal is by die casting. Die casting methods use liquid metal during casting and, as a consequence, molded metallic parts produced from this method can have low densities. Molded metallic parts having low densities are not generally desirable because of their reduced mechanical strength, higher porosity, and larger micro shrinkage. It is thus difficult to accurately dimension conventional molded metallic parts and, once dimensioned, to maintain their shapes. Moreover, molded metallic parts produced from conventional die casting have difficulty in reducing the resilient stresses developed therein.
Thixotropic methods for producing molded metallic parts generally improve upon the die casting method by injection molding a metal from its thixotropic (semisolid) state rather than from its liquid state. The result is a molded metallic part which has a higher density than one produced from the die casting method. Thixotropic methods are disclosed in U.S. Pat. Nos. 3,902,544 and 3,936,298, both of which are incorporated by reference herein.
Methods and apparatuses for manufacturing molded metallic parts from melted metal in its thixotropic state are also disclosed in U.S. Pat. No. 5,501,226 and Japanese patent publications 5-285626 and 5-285627, which are incorporated by reference herein. Methods of converting a metal into a thixotropic state by controlled heating and shearing in an extruder are disclosed in U.S. Pat. Nos. 5,501,226, 4,694,881 and 4,694,882. The systems disclosed in these patent documents are essentially in-line systems, in which the conversion of the metal alloy into a thixotropic state is assisted by an extruder and the pressurizing of the same for the purposes of injection molding; all these steps are carried out within a single cylindrical housing. It is difficult to accurately control all of the process parameters within a single cylindrical housing, especially temperature, shot volume, pressure, time, etc., and as a result, molded metallic parts of inconsistent characteristics are produced.
Moreover, some of these systems require that the metal supplied to the feeder be in pellet form. As a consequence, if a molded metallic part of undesired characteristics is produced by its system, recycling of the defective part is not possible unless the defective part is first recast in pellet form.
The present inventor's co-pending application Ser. No. 08/873,922, filed on Jun. 12, 1997, which is incorporated by reference herein, describes a different and improved method for producing molded metallic parts from melted metal in a thixotropic state wherein the conversion of melted metal into the thixotropic state takes place in a physically separate location from the location where the metal is injected into the mold and under different conditions.
An improved system for manufacturing molded metallic parts, which is capable of accurately producing molded metallic parts of specified dimensions within a narrow density tolerance that operates with melted metal in a thixotropic state, is desired. Further, a production process for molded metallic parts that can consistently produce molded metallic parts of desired characteristics and that can easily accommodate recycling of defective parts is desired. Further, an improved production process for molded metallic parts made of lighter metals, like magnesium, is desired.